You don't actually have to be on your own for the 300nm. I took a non-flying friend along for some company and made a day out of it. Break the trip up into 3 100nm legs and it's much more manageable.
I did North Weald - Welshpool - Gamston - North Weald. Got to see some interesting terrain and had a fantastic meal at Gamston and pottered round the Diamond hangers.